Communication system



Jan. 6, 1942. R. H. CAMPBELL COMMUNICATION SYSTEM 2 Sheets-Sheet 1 Filed June 28, 1940 Jan. 6, 1942. R. H. CAMPBELL COMMUNICATION SYSTEM Filed June 28, 1940 2 Sheets-Sheet 2 Patented Jan. 6, 1942 COMMUNICATION SYSTEM Richard H. Campbell, Racine, Win, assignor to Webster Electric Com poratlon of Delaware ram. R n Wis a cor- Application June 2:, 1940, Serial No. 342,836

The present invention relates to communication systems, and particularly to the provision of a new and improved circuit arrangement for indicating whether or not a station selected for communication is busy. Other inventions directed to other circuit arrangements for this and other purposes are disclosed and claimed in my contemporaneously filed applications having Serial Nos. 342,835 and 342,837.

It is an object of the present invention to'provide a new and improved communication system comprising a busy signal which is operatively controlled through the interstation communication circuit so that no additional interstation conductors are required.

A further object of the invention is the provision of an arrangement of the type described above wherein the busy signal is a normally energized device such as an electric light, whereby the light may be utilized also as a pilot light.

, A more specific object of the present invention is to provide a new and improved communication system comprising an arrangement whereby the busy signal at the calling station is operated by energy derived from a remote called station if that called station is busy.

Other objects and advantages of the present invention will become apparent from the ensuing description, in the course of which reference is had to the accompanying drawings, in which:

Fig. 1 represents schematically one embodiment of the invention; and

Fig. 2 represents a modification thereof.

In the modification of Fig. 1 there is illustrated an intercommunication system of the loudspeaking type, wherein communication 'is' carried on alternately in opposite directions between a plurality of master stations under the control of manually operable communication direction control switches at each station, and wherein communication may be carried on between a master station and one or more substations under the control of a communication direction control switch at the master station.

The master stations are indicated by reference characters I, 2 and 3, and the substation by reference character 4. These stations are provided with translating devices capable of being utilized both as microphones and receivers, such as the loudspeaker units II to I4, inclusive, which may be of the well-known permanent magnet type. The master stations I, 2 and 3 are provided with amplifiers I5, I6 and I1, communication direction control switches I8, I9 and 20, and station selector switches 2|, 22 and 23, all respectively.

The amplifiers may be of any well-known construction, and from the figure it may be seen that only the amplifier I5 at station I has been illustrated in detail. I In brief, the amplifier comprises an input transformer 24 and an output transformer 25, interconnected by two stages of amplification. Power is suplied to the amplifier from a suitable source of alternating current through a transformer 26 and in part through a rectifier tube 21. The supply of rectified current (the B supply) is directed to the amplifier through conductors 28' and 29', the former of which is connected directly to a common conductor 30 of the amplifier which is grounded, as indicated by reference character 3I, and the latter of which has interposed therein a power switch 32 operable simultaneously with the communication direction control switch I8. The B supply circuit to the amplifier is completed when the communication direction control switch is operated to condition the station for either supplied with heating current from a secondary winding 36 of the transformer 26, the midpoint of which is connected to the common ground conductor 30 by a conductor 38. v

The communication direction control switch I8 is a manually operable multi-pole three-position switch. 'In a first position (the up and idle position) the loudspeaker II is connected directly to a communication. circuit comprising conductors and 42 for the reception of calls from other stations provided with amplifiers; that is, from other master stations. In a second position (the intermediate and listening position in which the switch is illustrated) the loudspeaker is connected to the output and the communication circuit to the input of the amplifier. In a third position (the down and talking position, from which the switch is biased by a spring 43 into its intermediate position upon release), the loudspeaker is connected vto "the input and the communication circuit tothe output of the amplifier.

The connection of the loudspeaker to the communication circuit in the idle position of the communication direction control switch is as. 5 follows:

From conductor 48 through switch blade 44 and conductor 46, and from conductor 42 through switch blade 48 and conductor 88. In passing, it may be noted that in the idle position of the communication control switch the B supply to the amplifier is disconnected by reason of the fact that the circuit through conductor 28 is interrupted by switch blade 82.

The connection of the loudspeaker to the output and the communication circuit to the input of the amplifier in the intermediate position of the communication direction control switch is effected in the following manner: The loudspeaker II is connected to the output of the amplifier through conductor 46, switch blades 82, 54, conductor 56, resistor 88, and conductor 68 leading to one terminal of the secondary winding 62 of outputtransformer 25, and thence back to the loudspeaker through conductor 64, branch conductor 66, switch blades 68 and I8, and conductor 58. At the same time resistor I2 is connected in parallel with loudspeaker II in order to maintain substantially constant the impedance of the receiving circuit through a conductor I4 connected to conductor 56 and resistor 58, conductor I6, and switch blade 18. The connection of the communication circuit to the input of the amplifier is effected through switch blade 44, conductor 88 leading to one terminal of the primary winding 82 of the input transformer 24 and from the other terminal of this primary winding through conductor 84 and switch blade 48 to conductor 42.

The connection of the loudspeaker to the input and the communication circuit to the output of the amplifier in the third position of the communication direction control switch is effected in the following manner: The loudspeaker is connected to the input through conductor 46, switch blades 54 and 52, and conductor 88 leading to one terminal of the primary winding 82 and from thence back to the other terminal of the loudspeaker through conductor 84, switch blades I8 and 68, and conductor 58. The communication circuit is connected to the output of the amplifier through switch blade 44, branch conductor 86, and conductor 68 leading to one terminal of the secondary winding 62 and from thence to the other communication conductor 42 through conductor 64, branch conductor 88 and switch blade 48.

In order to avoid undesirable clicking noises while reversing the connections of the loudspeaker and communication circuit to the input and output of the amplifier, there is provided a switch blade 98 and an elongated contact 82 connected, respectively, to the output conductors 64 and 68 so as to short-circuit the output of the amplifier when the communication direction control switch is operated from its listen to talk position, or vice versa.

One of the important features of the present invention resides in the provision of a busy signal device. In the instant embodiment, the signal device is an electric light, the lights at stations I, 2 and 8 being indicated by reference characters 94, 86 and 98, respectively. The light 84 at station I is normally energized whenever the transformer 26 is connected to a source of alternating current so that the light acts also as a pilot light. Control of the energization of the light to perform the signalling function will be described in greater detail hereinafter. The supply of energy to the light is controlled by a switch I88 operable by a normally de-energized solenoid I82. when the solenoid I82 is de-energized, the light 84 is connected across a portion of the secondary winding 88 of the transformer 28 through a circuit including ground connection I84, switch I88 in its closed position, and conductor I88 leading to one terminal of the transformer, and the circuit is completed through conductors 88, 88 and ground connection 8|.

' The control of the solenoid is effected in part through a conductor I88 leading from the solenoid to the communication control switch, where it is adapted to be engaged by a switch blade I I8 when the communication direction control switch is in its uppermost or idle position. The solenoid is connected to groundby conductor H2 and is shunted by a condenser H4. The switch blade H8 is connected by a conductor II6 to a point between a pair of resistors I I8 and I28 connected in series across the conductors 48 and 42. In the listen and talk positions of the communication control switch, the switch blade II8 connects the point between resistors H8 and I28 to a source of power derived from the amplifier rectifier through a conductor I22, a series resistor I24, and conductor I26 leading to the active portion of conductor 28. Conductor I22 is connected to ground through resistor I28.

The selector switch H at station I includes a number of switches, as for instance the two indicated by reference character I82, which are the station number I switches and are ordinarily maintained in closed position in order that other stations may communicate with station I. Each of the master stations is provided with as many pairs of switches as there are stations in the system so that each station may communicate with the others. Selector switch 22 at station 2 is illustrated as having the same number of switches as selector switch 2I, and the switch blades I84 thereof are closed in order to connect station 2 to the system for the reception of calls from other stations. Selector switch 28 at station 8 has not been illustrated in full and is shown as including only the two pairs of switches I86 and I81, the former of which may be considered as being the switch associated with station 8, and the latter of which may be considered as associated with station 2.

Loudspeaker I4 at station 4 is connected to one pair of interstation conductors through conductors I88, so that communication may be had therewith whenever the proper. selector switches are operated at the master stations.

The apparatus at stations 2 and 8 corresponds identically withthat of station I, and for this reason it is not deemed necessary to describe the various circuit connections in detail. However, the signalling circuits to which the present invention primarily pertains will be described in order that a better understanding may be had thereof.

The communication circuit conductors at station 2 are indicated by reference characters I48 and 2. Connected across these in series are the resistors I44 and I46 and from between them extends a conductor I48 to switch blade I88 which, in the listen position in which it is indicated, and in the talk position, completes a circuit to the power supply associated with amplifier I6 through conductor I52 and resistor I64. Conductor I52, like conductor I22 at station I, is connected to ground through resistor I86. When the communication control switch I8 is operated in its uppermost or idle position, switch blade I58 connects conductor I48 to solenoid I88 through conductor m. Solenoid m is connected to ground through conductor I62 and is shunted by a condenser I64. Solenoid I60 is normally,

de-energized so that the contact I64 operated thereby is operative to connect light 86 to a transformer (not shown) similar to transformer 26 at station I through conductor I66 and the ground connection I68.

theoperator at station 8 closes selector I86. 1 This serves to eflect the connection otconductors I10 and, I12 across the conductors eitherstation I or station 2, depending upon the .The communication circuit at'station 8 comprises the conductors I10 and I12 across which are connected in series the resistors I14 and I16, from between which leads a conductor I18 to switch blade I80. In the indicated position of the switch blades and in the talk position, the blade completes a circuit to a source of energy through conductor I82 and resistor I84. Conductor I82 is grounded through a resistor I86. In the uppermost position of the communication control switch, switch blade I80 connects the point between resistors I14 and I16 to the solenoid I88 through conductor I90. Solenoid I88 is connected to ground through conductor I92 and is shunted by a condenser I94. Solenoid I88 is normally de-energized and when de-energized the switch contact I96 associated therewith serves to connect light 98 to a transformer (not shown) similar to that numbered 26 at station I through conductor I98 and ground connection 200. V

In the operation of the systemdescribed above in detail, it is contemplated that each of the amplifiers will normally be supplied with power from a suitable alternating current source through the transformers. Since the solenoid windings are normally de-energized, the pilot lights 94, 96 and 98 will normally be supplied with energy from the transformer winding 36 and similar transformers. The lights will, therefore, act as pilot lights in addition to acting as signalling devices.

The communication direction control switches at the various stations are normally in their idle positions, so that the solenoid windings are connected to points between the resistors connected in series across the individual communication circuits. When so connected, the windings are not supplied with energy, and, as a result, are de-energized as specified above.

Assuming now that the operator at station 2 desires to communicate with the operator at station I. The proper selector switch at station 2 is operated to connect conductors 40, 42 to I40, I42. This operation has no effect upon the energization of the solenoid windings, but as soon as the communication control switch at station 2 is operated from its uppermost to its intermediate or lowermost position, the midpoint between resistors I44 and I46 is connected to a source of power through conductor I52 and resistor I54, with the result that solenoid I02, which is connected between resistors H8 and 7 I by switch blade H0, is supplied with current and is energized; This results in the de-energization of light 94, giving an indication to the operator at station I that another station desires to communicate with station 'I. Thereafter, communication may be maintained between stations I and 2 simply by altemate'operation of the communication control switches thereat from their idle to their talk positions, or preferably, the communication control switch at the calling station is operated between its listen and talk positions.

Assuming further that the operator at station 3 desires to communicate with station 2, then mode ,of manipulation and the position of the communication control switches at these stations, through switch blade I in its uppermost position and resistors I14 and I16. Thus, the operator at station 8 is given a busy signal.

It station I is in communication with station 4 and the operator at station 8 selects either station I or station 4, the solenoid I88 at station 3 is energized to give the operator at sta tion .3. a busyv signal. This results from the fact that conductors 40 and 42 are supplied with energy from station I through switch blade H0. and resistors H8 and I20, and as soon as station I or station 4 is selected by station 3, the solenoid I88 is supplied with energy to energize the same.

Satisfactory results have been obtained in a system such as that described above where the voice coils associated with the loudspeakers have resistances of about 50 ohms; resistor 58 has a resistance of 25 ohms; resistor 12 has a resistance of 50 ohms; resistors I24, I54 and I84 have resistances of about 25000 ohms; resistors I02, I56 and I86 have resistances of about 10,000 ohms; and solenoids I02, I58 and I 88 have resistances of about 12,000 ohms.

In the modification of Fig. 2, the invention is shown applied to a slightly different type of intercommunicating system. In this system communication between master stations is carried onover two pairs of interstation circuits, one of which is used for communication in one direction and the other for communication in the opposite direction. The modification of Fig. 2 also difiers somewhat from that of Fig. I in that a slightly difierent form of communication control switch is illustrated. The communication direction control switch is of a type adapted to be operated from its intermediate and listen position to a lower and talk position and likewise to an upper and talk position. The switch is biased to return upon release from the lower talk position to the listen position, but remains in the upper talk position when actuated thereinto for the purpose of providing a system adapted particularly for use in installations where the communication i predominantly in one direction, as for instance between an executive and his secretary.

Referring now in detail to Fig. 2, it may be seen upon; inspection that the amplifier, the signal devices, and the control solenoids therefor, are identical with those of the previous modification. For this reason it is believed unnecessary to describe them in detail and identical reference characters have been applied to indicatethese portions of the equipment. As in the previous modification the illustrated system comprises four stations, I to 4, inclusive, each of which is provided with a loudspeaker indicated by reference characters I I to I4, respectively, and the master stations I, 2 and 3 are provided with communication direction control switches 22I, 222 and 223, respectively.

Referring now more particularly to the apparatus atstation I, it may be seen that the communication direction control switch thereof is a manually operable three-position switch comprising tour switch blades 224, 223, 228 and 236, respectively, all operable simultaneously and biased from the lowermost position to the intermediate position, in which it is shown, by a spring 232. In the indicated or listen position, the loudspeaker I I is connected to an interstation circuit comprising conductors 234 and 236, that is individual to station I. The connection to these conductors is effected through conductor 238, switch blade 226, a potentiometer connection 246, a volume control resistor 242 forming part of the potentiometer connection to one conductor and through conductor 244, switch blade 228, and conductor 246 to the other conductor. In this position of the communication direction control switch, the B supply to the amplifier is cut ofi by switch blade 224.

In the lower position of the communication direction control switch, the loudspeaker is connected to the primary winding 82 of the input transformer 24 of amplifier I5 through conductors 248 and 256 by means of the switch blades 226 and 228. At the same time the secondary winding 62 of the output transformer 25 of the amplifier is connected to another pair of interstation conductors 252 and 254 through conductor 256 and switch blade 236 and through conductor 258.

In the upper talk position the loudspeaker II is connected to the input of the amplifier through the same conductors and switch blades as before, and the output of the amplifier is likewise connected to the interstation conductors 252 and 254 in the same manner.

The output from amplifier I5 is adapted selectively to be connected to the other stations in the system, that is, to stations 2, 3 or 4, through the intermediary of selector switches, indicated generally by reference character 266 and comprising selector switches 262, 264 and 266, respectively, in a manner to be described in detail shortly.

The signal device controlling solenoid I62 is connected by conductor 212 to a point intermediate a pair of resistors 214 and 216 connected in series across the output conductors 252 and 254. The switch blades 268 and 216, which, are associated with the switches for selecting the other master stations 2 and 3 of the system, and switch blade 2" which is associated with the switch for selecting substation 4, are connected by conductor 218 to a point between resistors 286 and 282 connected in series across conductors 234 and 236. These switches serve to ground the point between the resistors through conductor 218 and. resistors 284, 286 and 288, respectively. The points intermediate the lastmentioned resistors and the switches associated therewith are connected through resistors 286, 292, and 294, respectively, to a source of energy (the active portion of conductor 29) through a conductor 296.

Communication direction control switches 222 and 223 at stations 2 and 3 are identical in all respects with the communication direction control switch 22I at station I,both as to the construction of the switch and the circuit connections therefrom to the loudspeaker, amplifier and interstation circuit. For this reason it is deemed expedient only to describe in detail the features of the signaling circuit to which the present invention pertains primarily.

Station 2 has associated therewith a pair 01 inter-station conductors 366 and 362 illustrated as connected to loudspeaker, I2. A pair of resistors 384 and 386 are connected in series across these conductors and from a point intermediate these there extends a conductor 338 to the grounding switch blades 3" and 3I2 associated with selector switches 8" and 3I3, respectively, by means of which the output of amplifier I6 may be directed to either station I or station 3. The ground connection is elected through resistors 3I8 and 326 associated with blades 3I3 and 3I2, respectively. The third selector switch for connecting the output 01 the amplifier to substation 4 has been omitted for the purpose of clarity and because of space requirements.

The output of amplifier I6 is led to the selector switches through a pair of conductors 326 and 322, across which is connected in series a pair of resistors 324 and 326. The solenoid winding 158 at station 2 is connected to a point intermediate these resistors by a conductor 328. The points between switch blade 3I6 and resistor 3I8 and switch blade 3I2 and resistor 326 are connected through resistors 336 and 332 and by a conductor 334 to a source of power associated with amplifier I6.

In identical manner the interstation conductors individual to station 3 indicated by reference characters 346 and 342 are connected thereacross in series by a pair of resistors 344 and 346. The point intermediate the two resistors is connected by a conductor 348 to switch blades 356 and 352 associated with the selector switches 3'54 and "356, respectively, by means of which the output of amplifier I1 may be directed to stations I' and 2. The switch blades 356 and 352 are adapted to complete connections to ground through resistors 358 and 366, respectively.

The output conductors 362 and 364 of amplifier I1 have connected thereacross in series the resistors 366 and 368. The point intermediate the two last-mentioned resistors is connected by a conductor 316 to the solenoid winding I66. The points intermediate switch blade 356 and resistor 358 and switch blade 352 and resistor 366 are connected to a source of power through resistors 312 and 314, respectively, and conductor 316 in the same manner as at the other stations.

In use it is contemplated that the communication direction control switches are left in the positions indicated, that is, in their listen positions, at which time the B supply to the amplifier is disconnected to render the amplifiers inoperative but ready for instantaneous use. The solenoids I62, I58 and I96 are tie-energized so that the light 94, 66 and 98 are supplied with power so that these lights. act both as signal and pilot lights.

Assuming that the operator at.station 2 desires to communicate withstation I, then the operator at station 2 operates the selector switch 3I4 to connect the output of amplifier I6 to the conductors 234, 236 and simultaneously to ground the point between resistors 364 and 366 through switch blade 3I6 and resistor 3I8. When the operator at station I responds, he operates the selector switch 262 thereat to connect the output of amplifier I5 to the communication circuit 366, 362 individual to station I. At the same time the midpoint between resistors 286, 282 is grounded through switch blade 268 and resistor 284. When both the selector switches have been operated into circuit establishing positions, the solenoid windings I62and I56 at stations I and 2 are supplied with energy from stations 2 and I,

respectively, and they are energized to disconnect the light 94 and 96 from their associated sources of energy.

When any other station, such for instance as station 3, attempts to call either station I or station 2 when the latter are in communicating relationship with each other, the light 98 thereat is extinguished. This results from the fact that the operation of either selector switch 354 or 356 into circuit establishing position results in the connection of solenoid winding I96 at station 3 to the source of power at either station I or station 2, whereby it is energized. Consequently, the operator at station 3 is apprised of the fact that the selected station is busy.

When master station I is in communication with a substation, such for instance, as substation 4, and another master station, for instance master station 2, selects station I, then the signal light 96 at station 2 is de-energized to indicate that master station I is busy. This results vfrom the fact that the midpoint between resistors 280, 282 at station I is grounded through switch blade 2' and resistor 288, whenever station I is in communication with station 4. Consequently, when selector switch 3 at station 2 is operated to select station I, then solenoid I56 at station 2 is supplied with energizing current from station I, and the operator at station 3 is apprised of the fact that station I is busy.

It is to be noted that while only a comparatively few stations have been shown in each of the embodiments of the invention illustrated, the number of stations may be multiplied to any extent desired. Although the invention has been disclosed with the specific details of preferred embodiments thereof, it should be understood that such details are not intended to be limitative of the invention except in so far as set forth in the accompanying claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. In a multiple station intercommunicating system, the combination including, signal means at each station, a source of energy at each station, a normally de-energized solenoid at each station controlling the signal means thereat, and means including a station selector switch at each station for supplying said solenoid with energy from a busy selected station.

2. In a multiple station intercommunicating system of the loudspeaking type, the combination including, an amplifier, signal means, signal control means, a communication control switch and station selector switches at each station, and interstation circuits, the communication control means at each station being so connected in circuit that they supply the interstation circuit with signal control means operating current when conditioning the stations for communication with each other, whereby whenanother station selects a station already in communication the signal control means thereat is supplied with energy to operate the signal to indicate that-the selected station is busy.

3. A unit for use in a multiple unit intercommunicating system, including in combination, signal means, a source of energy, a normally deenergized solenoid for controlling the signal means, and means including a, unit selector switch at each unit for conditioning said solenoid for receiving energy from a selected unit.

4. A unit for use in a multiple unit intercommunicating system, including a communication circuit, signal means, normally. de-energized signal control means, means for supplying said circuit with signal operating current when the circuit is conditioned for communication, and means connecting said control means to'said circuit whereby the control means is energized to operate the signal means upon selection of a unit in communication with another.

5. In a multiple station intercommunicating system'of the type wherein each station comprises an amplifier, a loudspeaker adaptedto be utilized both as a microphone and a receiver, a communication control switch operable in one position to connect the station for reception of calls from other stations, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches the combination including, signal means, normally de-energized signal controlling means, and switch means operable by said communication control switch when the latter is operated to said one position to connect said signal controlling means to said pair of wires.

6. A unit for use in a multiple station intercommunicating system wherein each unit comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and areceiver, a communication control switch operable in one position to connect the station for reception of calls from other stations, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches, including in combination, signal means, normally de-energized signal controlling means,

and switch means operable by said communication control switch when the latter is operated to said one position to connect said signal controlling means to said pair of wires.

7. In a multiple station intercommunicating system of the type wherein each station comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in one position to connect the station for reception of calls directly from other stations and in a second position for reception -of calls from other stations through the amplifier, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches, the combination including, signal means, signal controlling means, a source of current for said signal controlling means, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to said one position to connect said signal controlling means to a point between the resistors and to connect said point to said source of current in said second position, whereby when the communication control switch at one station is in said one position and another station selects that station and operates the communication control switch thereat to its second position the signal controlling means is energized to operate the signal to give an indication that another station is listening to theone station.

8. A unit for use in a multiple station intercommunicating system of the type wherein each unit comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in one position toconnect the unit for reception of calls directly from other units, and in a second position for reception of calls from other units through the amplifier, unit selector switches, and a single pair of wires leading from the communication control switch to the selector switches, including in combination, signal means, signal controlling means, a source of current, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to said one position to connect said signal controlling means to a point between said resistors and to connect said point to said source of current in said second position, whereby when the communication control switch of the unit is in said one position and another similar unit selects that unit and operates the communication control switch thereat to its second position the signal controlling means is energized to operate the signal to give an indication that another unit is listening to said unit.

9. In a multiple station intercommunicating system of the type wherein each station comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in its normal position to connect the station for reception of calls directly from other stations, in a second position for reception of calls from other stations through the amplifier, and in a third position for transmitting calls to other stations through the amplifier, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches, the combination including, signal means, signal controlling means, a source of current for said signal controlling means, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to its normal position to connect said signal controlling means to a point between said resistors and to connect said point to said source of current in its second and third positions, whereby when the communication control switch at one station is in its normal position and another station selects that station and operates the communication control switch thereat to its second position the signal controlling means is energized to operate the signal to give an indication that another station is listening to the one station, and when a third station selects one of two stations communicating with each other with the communication control switch at the third station in its normal position the signal controlling means at the third station is energized to indicate the selected station is busy.

10. A unit for use in a multiple station intercommunicating system of the type wherein each unit comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in its normal position to connect the unit for reception of calls directly from other units, in a second position for reception of calls from other units through the amplifier, and in a third position for transmitting calls to other units through the amplifier, unit selector switches, and a single pair of wires leading from the communication control switch to the selector switches, the combination including, signal means, signal controlling means, a source of current, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to its normal position to connect said signal controlling means to a point between said resistors and to connect said point to said source of current in said secmeans 0nd and third positions, whereby when the communication control switch at one unit is in its normal position and another unit selects that unit and operates the communication control switch thereat to its second position the signal controlling means is supplied with current to give an indication that another unit is listening to the unit, and when the unit selects a unit in communication with another with the communication control switch at the third unit in its normal position the signal controlling means at the third unit is connected to said source of current and energized to indicate that the selected unit is busy.

11. In a multiple station intercommunicating system of the type wherein first and second stations comprise an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in its normal position to connect the station for reception of calls directly from other stations, in a second position for reception of calls from other stations through the amplifier, and in a third position for transmitting calls to other stations through the amplifier, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches, and a third station comprises a loudspeaker adapted to be utilized both as a microphone and a receiver, the combination including, signal means, signal controlling means, a source of current for said signal controlling means, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to its normal position to connect said signal controlling means to a point between said resistors and to.

connect said point to said source of current in said second and third positions at said first and second stations, whereby when either said first or second station is communicating with the third station by operation of the communication control switch thereat to its second and third positions and the other of said stations selects either or the stations in communication with each other with its communication control switch in its normal position, the signal controlling means at the other station is connected to said source of current and energized to indicate that the selected station is busy.

12. In a multiple station intercommunicating system of the type wherein each station comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in its normal position to connect the station for reception of calls directly from other stations, in a second position for reception of calls from other stations through the amplifier, and in a third position for transmitting calls to other stations through the amplifier, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches, the combination including, a signal lamp, a source of current therefor, a normally closed switch for connecting said lamp to said source of current, a solenoid having one terminal grounded for controlling said switch, a pair of resistors connected across said wires, and means including a switch operable simultaneously with said communication control switch for connecting said solenoid to a point between said resistors when the communication control switch is in its normal position, a source oi current iorsaid signal operating means, said lastmentioned switch being adapted when the communication .control switch is in its second and third positions to connect the point between said resistors to said last-mentioned source of current, a resistor between said switch andsaid source of current, and means including a resistance for grounding a point between said last-mentioned resistor and the switch blade, whereby when the communication control switch at one station is in its normal position and another station selects that station andoperates the communication control switch thereat to its second position the solenoid is'connected to the source.of current at the selecting station and energized to disconnect the lamp from its source of current, and when a third station selects one of two stations communicating with each other with the communication control switch at the third station in its normal position, the solenoid at the third station is connected to the sources of current at one of the first and second stations and energized to disconnect the lamp at the third station from its source of current.

13. A unit for use in a multiple station intercommunicating system, including in combination, a communication circuit, an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in its normal position to connect the unit for reception of calls directly from other stations, in a second position for reception of calls from other units through the amplifier, and in a third position to transmit calls to other units through the amplifier, a signal lamp, a source of current therefor, a normally closed switch for connecting said lamp to said source of current, a solenoid having one terminal grounded for controlling said switch, a pair of resistors connected across said communication circuit, and means including a switch operable simultaneously with said communication control switch for connecting said solenoid to a point between said resistors when the communication control switch is in its normal position, a source of current, said last-mentioned switch being adapted when the communication control switch is in its second and third positions to connect the point between said resistors to said lastmentioned source of current, a resistor between said switch and said source of current, and means including a resistance for grounding'a point between said last-mentioned resistor and the Switchblade, whereby when the communica-. tion control switch at the unit is in its normal position and another unit selects the unit and operates the communication control switch thereat to its second position the solenoid is connected to the source of current at the selecting unit and energized to disconnect the lampifrom its source of current, and when a third unit selects one of two units communicating with each other with the communication control switch at the third unit in its normal position the solenoid at the third unit is connected to the source of current at one of the first and second units and energized to disconnect the lamp at the third unit from its source of current.

14. In a multiple station intercommu'nicating system of the type wherein each station comprises an incoming line, an outgoing line, and selector switches adapted to connect the outgoing line to other stations, the combination including, signal means, signal controlling means, sources of current for the signal controlling means, and switch means operable with the seaaoaua I 7 lector switches constructed and arranged that when two stations are connected for communication with each other the signal controlling means at the'stations are connected to sources of current at the other stations.

15. A unit for use in a multiple station intercommunicating system, including in combination, an incoming line, an outgoing line, signal means, signal cotrolling means connected to said outgoing line, a source of current, and switch means in said outgoing line for connecting the incoming line to said source 0! current when the unit is conditioned for communication with another unit.

16. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, and selector switches adapted to connect the outgoinglline to other stations, the combination including, signal means, signal controlling means, a source of current for said signal controlling means, a pair of resistors connected across the outgoing line, a connection from between said resistors to said signal controlling means, a pair of resistors connected across the incoming line,-

and means including switches operable with the selector switches for connecting a point between said last-mentioned resistors to said source of current whenever a selector switch is operated.

17. A unit for use in a multiple station intercommunicating system, including in combination, an incoming line, an outgoing line, selector switch means adapted to connect the outgoing line to other stations, signal means, signal controlling means, a source of current, a pair of resistors connected across the outgoing line, a connection from between said resistors to said signal controlling means, a pair of resistors connected across the incoming line, and means including a switch operable with the selector switch means for connecting a point between said last-mentioned resistors to said source of current whenever the selector switch is operated.

-l8. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, selector switches adapted to connect the outgoing line to other stations, the combination including, signal means, a, signal controlling solenoid having one terminal grounded, a source of current for said signal controlling means, a-pair of resistors connected across the outgoing line, a connection from a second terminal of said solenoid to a point between said resistors, a pair of resistors connected across the incoming line, and means including switches operable with the selector switches for connecting a point between said last-mentioned resistors to said source of current and to ground, said connections to the source of current and ground including resistors.

19. A unit for use in a multiple station intercommunicating system of the type wherein each unit'comprises an incoming line, an outgoing line,

selector switches adapted to connect the outgoingline to other. units, including in combination, signal means, a signal controlling solenoid having one terminal grounded, a source of current, a pair of resistors connected across the outgoing line, a connection from a second terminal of said solenoid to a point between said resistors, a pair of resistors connected across the incoming line, and means including switches operable with the selector switches for connecting apoint between said last-mentioned resistors to said source oi current and to ground, said connections to the source of current and ground including resistors.

20. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, selector switches adapted to connect the outgoing line to other stations, a loudspeaker adapted to be utilized both as a microphone and a receiver, an amplifier, and a communication control switch operable in one position toconnect the loud-speaker to the incoming line for the reception of calls from other stations, the combinationincluding,

signal means, signal controlling means, a source of current for said signal controlling means, and switch means operable with the selector switches so constructed and arranged that when two stations are connected for communication with each other the signal controlling means at each station is connected to the source of current at the other station.

21. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, selector switches in the outgoing line to connect the line to other stations, a loudspeaker adapted to be utilized both as a microphone and a receiver, an amplifier, and a communication control switch operable in one position to connect the loudspeaker to the incoming line for reception of calls from other stations, the combination including, signal means, signal controlling means, a source of current for said signal controlling means, a pair of resistors connected across the outgoing line, a connection from between said resistors to said signal controlling means, a pair of resistors connected across the incoming line, and means including a switch operable with the selector switch for both grounding and connecting to said source of current a point between said last-mentioned resistors whenever the selector switch is operated to select a station.

22. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, selector switches in the outgoing line for connecting it to other stations, 9. loudspeaker adapted to be utilized both as a microphone and a receiver, an amplifier, and a communication control switch operable in one position to connect the loudspeaker to the incoming line for the reception of calls from other stations, and in another position to connect the loudspeaker to the input of the amplifier and the outgoing line to the output of the amplifier for thetransmission of calls to other stations, the combination including, signal means, signal controlling means, a source of current for said signal controlling means, a pair of resistors connected across the outgoing line, a connection from between said resistors to said signal controlling means, a pair of resistors connected across the incoming line, means including resistors and switches operable simultaneously with the selector switches for grounding through said resistors a point between the resistors connected across the incoming line and for connecting said point to said source of current, whereby when two stations are placed in communication with each other the signal controlling means at said stations are energized, and when a selector switch of a third station is operated to select a busy station the signal controlling means at the third station is energized.

23. A unit for use in a multiple station intercommunicating system, including in combination, an incoming line, an outgoing line, signal means, signal controlling means, a source of current, a pair of resistors connected across the outgoing line, a point between which is connected to the signal controlling means, a pair of resistors connected across the incoming line, unit selector switches in the outgoing line, switches operable simultaneously with the selector switches for grounding a point between the last-mentioned resistors and for connecting the point to said source of current, whereby the signal controlling means is energized when said unit is placed in communication with another and when said unit selector switches are operated to select a busy unit.

24. In a multiple station intercommunicating system of the type wherein a station comprises an incoming line, an outgoing line, selector switches in the outgoing line to connect the line to other stations, a loudspeaker adapted to be utilized both as a microphone and a receiver, an amplifier, and a communication control switch operable in one position to connect the loudspeaker to the incoming line for reception of calls from similar stations and in a second position for connecting the loudspeaker to the input of the amplifier and the output of the amplifier to the outgoing line, and wherein another station comprises a loudspeaker utilizable both as a microphone and a receiver, the combination including, signal means, signal controlling means, a source of current for said signal controlling means, a pair of resistors connected across the outgoing line, a connection from therebetween to the signal controlling means, a pair of resistors connected across the incoming line, a switch operable simultaneously with the selector switch for grounding and connecting to said source oi! current a point between the last-mentioned pair of resistors, whereby the signal controlling means at other stations similar to the first are connected to said source of current to operate the signal means upon selection of the first station.

RICHARD H. CAMPBELL. 

